Tobacco smoke filter

ABSTRACT

A tobacco smoke filter is in the form of a cylinder having a low resistance core and a high resistance end wall which cause smoke flow radially through the cylinder wall. The filter is preferably made by compacting the central core of a filter plug to form the dense end wall. Such filter plugs have an efficiency of 2 or 3 times that of a conventional filter of the same pressure drop.

United States Patent Westcott et a1.

TOBACCO SMOKE FILTER Inventors: David Thomas Westcott; Geoffrey Roger Woodman, both of Bristol, England Imperial Tobacco Group Limited, London, England Filed: Aug. 31, 1973 Appl. No: 393,591

Related US. Application Data Division of Ser. No, 217,797, Jan. 14, 1972, Pat, No, 3,791,265.

Assignee:

Foreign Application Priority Data Feb 1. 1971 United Kingdom H 3550/71 US. Cl. 93]] C; 93/77 FT; 131/261 B Int. Cl. A24c 5/50 Field of Search 93/1 C, 77 FT; 131/261 R,

131/261 A, 261 B, 10.5, 11, l2, 13, 14

References Cited UNITED STATES PATENTS 11/1967 Hall 93]] C X 1 June 10, 1975 Dock 131/261 A Labbe 131/261 B FOREIGN PATENTS OR APPLICATIONS 1,086,443 10/1967 United Kingdom 11 131/14 278.534 10/1927 United Kingdom 131/13 Primary Examiner-Roy Lake Assistant Examiner-James F. Coan Attorney, Agent, or FirmLarson, Taylor & Hinds 8 Claims, 15 Drawing Figures PATENTEDJUHIO 1915 TOBACCO SMOKE FILTER This is a division of application Ser. No. 217,797, filed Jan. l4, I972, now US. Pat. No. 3,79l,265.

This invention concerns a filter for tobacco smoke, cigarettes made therewith and a process and apparatus for making such filters. The term cigarette where used throughout this specification is intended to include within its scope not only conventional cigarettes but also cigars, cigarlettes and cigarillos.

According to one aspect of the present invention there is provided a method of making a tobacco smoke filter for a cigarette comprising providing a cylinder of filter material and compacting a core section of the cylinder to provide an end wall which is denser than the cylindrical wall of the filter.

Preferably the method comprises providing a double length filter rod, compacting a core section of the rod from both ends simultaneously to provide a central plug section denser than that of the cylindrical and separating the rod transversely at the central plug section to provide two hollow cylindrical filters each with one denser end wall.

According to another aspect of the present invention, there is provided apparatus for forming a filter plug having a compacted core section comprising a fluted drum for holding filter plug blanks thereon, a compacting rod aligned with each flute and adapted to reciprocate relative to the flute, and means for reciprocating each rod relative to each flute during rotation of the drum to compact a core section of a filter plug blank held in a flute of the drum.

According to another aspect of the present invention there is provided a tobacco smoke filter for a cigarette comprising a cylinder of filter material the interior of which provides a fiow path of lower resistance to flow than the wall of the cylinder, one end of the cylinder being sealed with a tranverse wall memberwhich presents a higher resistance to flow than the wall of the cylinder, whereby, in use, flow of smoke purely axially of the filter will be inhibited, causing smoke flow through the cylinder wall.

The invention also includes a cigarette comprising a tobacco rod and a hollow cylindrical filter having at the mouth end an end wall of material denser than the cylindrical wall of the filter, and smoke channels between the external surface of the filter and wrapper, whereby in use tobacco smoke passing into the interior of the filter from the tobacco rod will pass through the cylinder wall and through the smoke channels to the mouth end of the cigarette.

The invention is illustrated merely by way of example in the accompanying drawings in which,

FIG. 1 is a longitudinal cross-section through a conventional filter plug,

FIG. 2 is a longitudinal cross-section through a filter plug according to the present invention,

FIGS. 3 and 4 illustrate one method providing a filter plug as shown in FIG. 2,

FIGS. 5 and 6 illustrate a further method of providing the filter plug of FIG. 2,

FIG. 7 illustrates an alternative way of effecting the method illustrated by FIGS. 5 and 6,

FIG. 8 is a part sectional elevation of one form of apparatus for making filter plugs according to the method described with reference to FIG. 7,

FIG. 9 is a cross-section on line lXIX of the apparatus shown in FIG. 8,

FIGS. 10a and 10b are an end cross-section and a lateral cross-section respectively through a cigarette including a filter according to the present invention,

FIGS. 11a and llb are an end cross-section and a lateral cross-section respectively through a further cigarette incorporating a filter according to the present invention, and

FIGS. 12a and 12b are an end cross-section and a lateral cross-section respectively through yet a further cigarette incorporating a filter according to the present invention.

Referring to the drawings, and in particular FIGS. 1 and 2, there is shown in FIG. 1 a conventional filter plug 2 of cylindrical form and made of cellulose acetate fibre. In a typical plug the dimension 0, which is the length of the cylindrical plug adapted to be disposed longitudinally of the cigarette rod is 15 mm and dimension b which is the diameter of the cylindrical plug, is 8 mm. With the filter plug, tobacco smoke passes from one end face of the cylinder to the other end face thereof longitudinally of the cylindrical plug, as illustrated by arrows p. The pressure drop of a typical cellulose acetate filter plug of these dimensions is 2V2 cm of water and the efficiency of a typical plug is 30% Le. 30% of all particulate matter carried by the smoke passing through such a plug is retained within the filter plug.

Referring now to FIG. 2 there is shown a cellulose acetate filter plug 3 according to the present invention comprising a hollow cylindrical member 4 sealed at one end with an end plug which is denser than the wall 4 and which blanks off one end of the hollow core. For comparison purposes, this plug is of the same overall dimensions as plug 2 where a is 15 mm. b is 8 mm., and furthermore c is 4 mm, d is 2 mm. and e is 4 mm. Because the density of the plug 5 is greater than the density of the hollow cylindrical wall 4 the flow path adopted by the tobacco smoke passing through this filter will be transversely of the cylindrical wall 4 as indicated by arrows q. The smoke will not pass through the end plug 15 in passing through the filter but will take the path of least resistance.

With a filter plug of the type shown in FIG. 2, a typical pressure drop has been found to be l.8 cm. of water with an efficiency of 35%. Thus compared with the conventional plug shown in FIG. 10 substantially lower pressure drop is experienced whilst an increase in efficiency is achieved.

The reason for these beneficial results is thought to be the increased transverse area of filter plug material presented to the path of the tobacco smoke. Thus the area of material presented to the tobacco smoke in the plug 2 of FIG. 1 is that of a disc of diameter b (16 1r sq.mm.) whereas in FIG. 2 the area presented to the tobacco smoke is that of the surface of a cylinder of diameter c and length u-c. (44 1r sq.mm.). The substantial increase in cross-sectional area of filter material presented to the tobacco smoke (a factor of 2.75) accounts for the beneficial results achieved by decrease in pressure drop across the filter and/or increase in effciency.

FIGS. 3 and 4 illustrate one way of forming a filter plus according to the present invention. In FIG. 3 a cylindrical plug 10 of cellulose acetate length 15 mm. diameter 8 mm., is supported upon a solid surface 6. A

cutting tool 11 of diameter somewhat less than 4 mm. is plunged into the cylindrical rod for a distance of l 1 mm. whereby a core of the cellulose acetate is cut from the cylinder and. as it is cut it is compacted or compressed by the tool 11 to form a very dense wall 13 at the lower end of the cylinder as shown in FIG. 4. If tearing is experienced in the cutting action. the use of a warm tool can assist in providing a clean cut.

The resulting plug 12 is in the form of a hollow cylinder having a cylindrical wall 14 of cellulose acetate having the same density as that of the filter plug from which it was formed, and an integral end wall or plug 13 of a density which is greater than that of the wall 14. The core 15 of the filter plug is hollow. By forming the integral wall or end plug 13 denser than the wall 14 and since and wall 13 is sealed to the cylindrical wall 14 and inhibits leaks, smoke is forced radially through the wall 14 of the cylinder in passing through the filter.

Referring to FIGS. 5 and 6 there is illustrated a further method of providing a filter plug as shown in IFIG. 2. In this embodiment a cylindrical cellulose acetate blank being l5 mm. long and 8 mm diameter and from which the filter plug is to be formed includes an external cylindrical rod 21 of cellulose acetate packed at a density similar to that of the plug 10 of FIG. 3 (ie at the density required for the wall of the final cigarette filter). Within the cylindrical rod 21 is a core 22 of 4 mm diameter made from cellulose acetate of a density less than that of the wall 21 and thus more easily com pressible. Such a blank may be conveniently formed by co-extruding the core 22 of less dense cellulose acetate with the surrounding cylindrical rod 21 of more dense cellulose acetate.

To form a filter plug such as that shown in FIG. 2 the blank 20 of FIG. 5 is placed upon a solid support 19. A piston 23 as shown in FIG. 6 and of diameter some what less than 4 mm is plunged through the core to a depth of l l mm to compress or compact the core 22 into a dense end plug 24 of 4 mm thickness as shown in FIG. 6. It will be appreciated that the cylindrical wall 21 will remain unaffected by this operation and will be of the same density as the material forming the plug. However the end plug 24 is arranged to be of a density greater than that of the wall 21. Since the core 22 is distinct from and not integral with the cylindrical wall 21 of blank 20 no cutting action is required in forming the plug shown in FIG. 6, and a simple plunging action is all that is required. Such an arrangement is simpler than that shown in FIG. 4 since no tearing is experienced and a cleaner and a more satisfactory plug is achieved. Sealing of the end plug is achieved in this embodiment by an expansion of the plug diameter during the compression longitudinally. thus sealing the plug within the cylindrical wall 21.

In the embodiment described with reference to FIGS. 5 and 6 the core material 22 need not be the same as that forming the wall 21. Although in the preferred embodiment described above both materials are cellulose acetate of different packing densitities, it will be appreciated that the core material could be different from that forming the wall 21. Of course neither need be cellulose acetate. Thus suitable materials could be a foamed synthetic resin or another synthetic fibre.

Referring to FIG. 7 there is shown yet another method of achieving a filter plug according to the present invention. The method is similar to that shown in FIGS. 5 and 6 (but can be adapted to be similar to that shown in FIGS. 3 and 4) but in which a double length filter rod is plunged from both ends simultaneously whereby a central dense wall 30 is formed with two hollow cores 31 and 32. After the plunging operation, the filter plug is separated be severing at 33 to form two filter plugs in accordance with the present invention. The preferred method of achieving this dual length filter is in accordance with the method described with reference to FIGS. 5 and 6 since no cutting or tearing action is necessary. The cutting action of FIGS. 3 and 4 can however be employed if required. It will be appreciated that no lower support is required for the filter plug during the plunging operation since the opposed forces of the two plungers 34 will balance.

One form of apparatus suitable for producing a dual length filter plug similar to that shown in FIG. 7 is shown in FIG. 8. This apparatus comprises a mounting plate 60 attached to the frame of a machine and having mounted thereon a cylindrical housing 61 with fixed end plate 62 into the centre of which is rigidly fixed by nuts 63 one end of a stationary shaft 64. To the other end of the stationary shaft 64 is secured a stationary cam plate 65, a key 66 and nut 67 being employed to secure the cam plate 65 to the shaft 64.

Fixed to the inner face of mounting plate 60 is an annular cam plate 70. A dowel pin 71 locates the cam plate relative to the mounting plate 60 and a number of studs 72 secures the cam plate 70 to the mounting plate 60. A cam surface 73 on the inner face of plate 70 extends adjacent the periphery of cam plate 70 starting at the top dead centre of the cam plate (i.e. at location M on FIG. 9) with zero rise and continuing to the bottom dead centre position (i.e. at location N in FIG. 9).

The maximum rise, at position N, is indicated by boss 74 in FIG. 8. At this point the cam is stepped, the remainder of the cam plate from N to M (travelling anti clockwise) being planar with no cam surface.

Cam plate 65 is similarly provided on its inner face with a cam surface 79 identical in form to cam surface 73. Cam surface 79 rises to a maximum from position M to N and is there stepped to zero for the remainder of the plate.

Also mounted on the inner face of the stationary cam plate 65 is a stationary annular carbon pad 77 provided on its inner face with an annular groove 78. A pipe 80 connected to a source of vacuum or suction (not shown) communicates with the annular groove 78 at position M.

Mounted upon the stationary shaft 64 by needle bearings 81 and 82 is a drive shaft 83 provided at one end with a drive gear 84 connected to a drive source (not shown). Keyed by key 85 to the drive shaft 83 is a composite solid drum formed primarily of three disc segments 86, 87 and 88 fixed rigidly together (by means not shown) and doweled by dowel pin 90 to ensure accurate relative location. The central disc 87 is provided at its periphery with twelve equi-spaced semi cylindrical flutes or slots 91 (see FIG. 9) each of the correct length and diameter to accommodate a dual length filter plug blank 99 e.g. 30 mm long and 8 mm diameter. Communicating with each slot 91 is a plurality of channels 92 which in turn communicate with a common air duct 93 which extends through disc 88 and seals against groove 78 in the annular carbon pad 77. The annular groove 78 in the annular carbon pad 77 extends from immediately before top dead centre position i.e. from position N as shown in FIG. 9 until the bottom dead centre position i.e. position N, extending in an anti-clockwise direction as indicated by arrow P. For the remainder of the circumference i.e. from N to L channel 78 is blanked off. Thus vacuum will be ap plied through the channels 92 to the semi cylindrical flutes or grooves 91 from location L until location N in order to secure and hold dual length filter plug blanks within the grooves during rotation of the drum. A hopper (not shown) disposed with its outlet between locations L and M as viewed in FIG. 9 is arranged to deposit dual length filter plug blanks into the grooves 91 as they pass the location between L and M as disc 87 rotates.

Disposed in each of the discs 86 and 88 are 12 equispaced plungers 100, 101 respectively. The plungers are housed within holes 102 and 103 respectively and in discs 86 and 88. The ends of the plungers project to the cam plates 70 and 65 respectively and bear against the cam surfaces 73 and 79. The plungers 100 and I] are urged into contact with cam plates 70 and 65 by springs I04 and I05 respectively. Cylindrical tools 106 and 107 slide within locating holes provided in annular guide plates 108 and 110 between the full line positions and the broken line positions shown in FIG. 8 as the cylinder moves from the top dead centre position N illustrated in FIG. 9 to the bottom dead position N illustrated in FIG. 9. For a 30 mm. plug blank the tools are arranged to penetrate the ends of the blank for a distance of say I 1 mm. leaving an 8 mm dense wall centrally of the plug. When the 30 mm plug is severed into two mm plugs the central core will form a 4 mm dense end wall for each plug in the manner described with reference to FIG. 7.

In use, dual length filter plug blanks are fed into slots 91 between locations L and M as seen in FIG. 9, suction being applied to the slots 91 between the position L and N whereby the filter plug blanks are held in the grooves. As the cylinder rotates from location M to N (in an anti-clockwise direction as viewed in FIG. 9) the plungers 100 and 101 will be cammed inwardly of the filter plug blank whereby the tools 196 and 107 will compact the core of the blank from either end to form a central dense end wall. At the location N the cam surfaces are stopped whereby the plungers spring back to their retracted (full line) positions withdrawing the tools from the filter plugs. Also at location N, the suction to grooves 91 is cut off. Immediately after location N the compacted plugs are free of the grooves 91.

Although as described, the cam surfaces and vacuum source are applied to the drum for more than half its revolution, it will be appreciated that either or both these actions could be operable for a shorter or longer period. depending upon the desired operation of the equipment.

The drum described above with reference to FIGS. 8 and 9 preferably forms part of a filter tip cigarette mak ing machine and may for example replace the filter drum on a Hauni KFZ filter cigarette making machine. In such a machine the compacted dual length filter rods leaving the drum between locations N and Q as seen in FIG. 9 will be transferred to a drum containing two cigarette lengths located on either side of each dual length filter plug; these will be rolled in a paper wrapper to form two filter cigarettes joined end to end and will subsequently be severed to form two single filter tip cigarettes.

The filter plugs made in accordance with the present invention can be used in a variety of ways. It will be appreciated that the plug having one end blocked by a wall will present one flat surface which can be placed at the mouth end of the cigarette to provide a pleasing appearance. In this event the open end of the cylindri' cal plug will abut the tobacco rod of the cigarette. Alternatively, the open end of the filter rod can be at the mouth end of the cigarette the plug of the filter rod then being in abutment with the tobacco rod of the cigarette.

To achieve maximum efficiency from filters according to the present invention it is desirable that tobacco smoke passing through the walls of the filter should be permitted to pass radially through the walls rather than be forced to pass longitudinally through the walls. Should the filter plugs be wrapped by impermeable paper or relatively impermeable paper, as in conventional cigarette. the pressure drop of the filters may well be far in excess of that which is normally accept able to the smoker since smoke will be forced to travel longitudinally through the wall of the filter.

To enable smoke to pass through the cylindrical wall radially of the plug, air ducts or passages must be provided externally of the filter plug through which the smoke can pass. Three ways in which this can be achieved are shown in FIGS. 10, 11 and 12.

Referring to FIG. 10 there is shown the mouth end of a cigarette including a filter plug 35 in accordance with the present invention. Plug 35 has a cylindrical wall 40 and a denser end wall 51. The plug is wrapped by a dual layer paper wrapper comprising an external cylindrical sheath 4] and an internal corrugated sheath 42. The internal corrugated sheath 42 contacts both the external sheath 4! and the external surface of the cylindrical wall 40. Thus air channels are provided between the corrugations of the sheath 42 and the external surface of the cylindrical wall 40. Smoke which passes through the core of the filter 35 passed radially outwardly through wall 40 and to the smokers mouth along these longitudinal ducts or air channels.

Referring to FIG. 11 there is shown an alternative embodiment including a filter plug 36 according to the present invention comprising a cylindrical wall 45 and a denser end wall 52. The plug is wrapped by a single corrugated or longitudinally fluted cigarette wrapper 46. Again, air passages are provided between the flutes of the wrapper 46 and the external surface of the cylindrical wall 45 providing passageways for the smoke passing through the wall of the filter to pass to the smokers mouth. Such an arrangement of fluted or corrugated external wrapper for a cigarette also provides less surface contact between the cigarette wrapper and the smokers lips preventing sticking and also provides passage in the external corrugations of the paper in which ambient air can be drawn into the smokers mouth.

Referring now to FIG. 12 there is shown yet another arrangement including a filter plug 37 in accordance with the present invention comprising a cylindrical wall 48 and a denser end wall 53. The plug is provided with an external longitudinally fluted surface 49 and this is wrapped by a cylindrical wrapper 50. Again, longitudinal air channels are provided between the external flutes of the surface 49 of wall 48 and the wrapper 50 allowing smoke to pass to the smokers mouth.

A method of making the fluted filter plug blanks from which elements 48 can be formed is described and claimed in cognate U.K. Patent applications No. 8462/70 and 23854/70.

it will be appreciated that, in the embodiments of FIGS. 10-12, the filter plug could in each instance be reversed to place the end walls or plugs 51, 52 and 53 respectively in abutment with the tobacco rods 54, 55 and 56 respectively in which case an open cylindrical end would be presented to the smokers mouth. The open end could be blocked by an end plug but, unlike the relatively dense plug 51, 52 or 53, this plug would be provided morely to close the end of the cylinder and would be of a material providing a less dense surface than that of the wall of the cylinder of the filter plug thus inducing the smoke to fiow through this plug into the smokers mouth.

it will also be appreciated that the filter plugs according to the present invention need not be hollow but could be packed with a filter material which is less dense than that of the cylindrical wall. Nevertheless the cylindrical plug must still be provided with an end wall which is denser than that of the wall of the plug thus inhibiting flow and forcing the tobaccco smoke to pass radially through the wall of the cylinder.

The material from which filter plugs according to the present invention are made is not important although cellulose acetate and foamed and other fibrous synthetic resins are preferred.

in view of the difficulty of wrapping filter plugs with paper having convolutions or corrugations or flutes therein as shown for example in the embodiments of FIGS. 10 and 11, the preferred embodiment according to the present invention is that shown in FIG. 12 in which the longitudinal flutes or corrugations are already extruded within the cellulose acetate rod forming the filter plug and this is merely wrapped in a conventional cylindrical sheath. Alternative arrangements can of course be provided. Thus the external surface of the filter could be dimpled or otherwise roughened merely to provide proturberences which space the overlying wrapper from the surface of the filter plug thereby providing air passages to enable the smoke to pass to the smokers mouth.

Test results with filter plugs according to the present invention indicate that more efficient plugs are provided even with pressure drops the same as these of conventional plugs. Cellulose acetate plugs of the type described with reference to FIG. 7 but of 40 mm. over all lengths were compacted to a core depth of 15 mm. either and, leaving a central wall of 10 mm. These plugs were cut into two hollow cylindrical sections, each mm. long with 5 mm. dense end wall and were tested for pressure drop and efficiency. The results of three different typical samples, having different core densities before plunging, are quoted in Table 1.

in each sample the outer cylinder density was the same as that ofa conventional plug of cellulose acetate. The results tabulated are:

a. Density of inner core before compacting (gm. per. cc.)

b. Pressure drop of plug after compacting (cm. of water).

c. Efficiency of compacted plugs d. Efficiency of conventional plug of same pressure drop Similar tests on conventional plain cellulose acetate plugs of the same pressure drops are also quoted, from which it will be seen that a considerable increase in efficiency is obtained with the compacted rods.

Thus with filter plugs in accordance with the present invention it is envisaged that much more efficient plugs can be provided but having the same pressure drop as those conventionally provided with present day filter plugs. By increasing the pressure drop of the filter, yet further increases in efficiency can be achieved. Rather than make compacted plugs having the same pressure drop as conventional plugs, it is possible to make plugs of the same efficiency, in which case the compacted plugs will have a lower pressure drop compared with conventional plugs.

Due to the increase in efficiency it would be possible for example to replace current dual-plug filters e.g. of Myria (MYRlA is a Registered Trade Mark) and cel lulose acetate, with single plug filters made of one material, thus providing considerable saving in cost and handling.

Substantial decreases in the pressure drop of the plug as a whole and/or substantial increases in efficiency can be achieved with filters according to the present invention compared with conventional filters of the same material and external dimensions.

What we claim is:

l. A method of making a selectively compacted tobacco smoke filter comprising providing a cylinder of filter material and compacting only a core section of the cylinder, leaving an annular uncompacted wall extending around the compacted core section.

2. A method as claimed in claim 1, comprising providing a double length cylinder of filter material, compacting a core section only of the cylinder from both ends simultaneously to provide a central plug section denser than that of the cylindrical wall and separating the cylinder transversely at the central plug section to provide two hollow cylindrical filters each with an annular uncompacted wall extending around the compacted core section.

3. A method of making a selectively compacted tobacco smoke filter comprising providing a cylinder of filter material having a separate core of filter material therein and compacting said separate core of filter material, leaving an annularly uncompacted wall extending around the compacted core section.

4. A method as claimed in claim 3, wherein the cylinder and core are of differing densities.

5. A method of making a selectively compacted tobacco smoke filter comprising providing a cylinder of filter material in the form of a homogeneous rod, compacting only a core section of the cylinder, leaving an annular uncompacted wall extending around the compacted core section and simultaneously cutting or shearing a core from said rod as the core is completed.

6. A method of making a selectively compacted tobacco smoke filter comprising providing a double length cylinder of filter material having a separate core of filter material therein, compacting said separate core section only of the cylinder from both ends simultaneously to provide a central plug section denser than that of the cylindrical wall and separating the cylinder transversely at the central plug section to provide two hollow cylindrical filters each with an annular uncompacted wall extending around the compacted core section.

7. A method as claimed in claim 6, wherein the cylinder and core are of differing densities.

core is compacted. 

1. A method of making a selectively compacted tobacco smoke filter comprising providing a cylinder of filter material and compacting only a core section of the cylinder, leaving an annular uncompacted wall extending around the compacted core section.
 2. A method as claimed in claim 1, comprising providing a double length cylinder of filter material, compacting a core section only of the cylinder from both ends simultaneously to provide a central plug section denser than that of the cylindrical wall and separating the cylinder transversely at the central plug section to provide two hollow cylindrical filters each with an annular uncompacted wall extending around the compacted core section.
 3. A method of making a selectively compacted tobacco smoke filter comprising providing a cylinder of filter material having a separate core of filter material therein and compacting said separate core of filter mateRial, leaving an annularly uncompacted wall extending around the compacted core section.
 4. A method as claimed in claim 3, wherein the cylinder and core are of differing densities.
 5. A method of making a selectively compacted tobacco smoke filter comprising providing a cylinder of filter material in the form of a homogeneous rod, compacting only a core section of the cylinder, leaving an annular uncompacted wall extending around the compacted core section and simultaneously cutting or shearing a core from said rod as the core is completed.
 6. A method of making a selectively compacted tobacco smoke filter comprising providing a double length cylinder of filter material having a separate core of filter material therein, compacting said separate core section only of the cylinder from both ends simultaneously to provide a central plug section denser than that of the cylindrical wall and separating the cylinder transversely at the central plug section to provide two hollow cylindrical filters each with an annular uncompacted wall extending around the compacted core section.
 7. A method as claimed in claim 6, wherein the cylinder and core are of differing densities.
 8. A method of making a selectively compacted tobacco smoke filter comprising providing a double length cylinder of filter material in the form of a homogeneous rod, compacting a core section only of the homogeneous rod from both ends simultaneously to provide a central plug section denser than that of the cylindrical wall, separating the cylinder transversely at the central plug section to provide two hollow cylindrical filters each with an annular uncompacted wall extending around the compacted core section and simultaneously cutting or shearing a core from said rod as the core is compacted. 